1. Field of Invention
This invention relates to apparatus for detecting and amplifying weak impulses of energy absorbed from incident x-ray photons, gamma ray photons, or nuclear charged particles.
2. Discussion of Prior Art
Semiconductor junction diodes, employed alone or combined with (i.e. optically coupled to) a scintillating crystal, are used as transducers to convert impulses of energy absorbed from incident x-ray photons, gamma ray photons, or nuclear charged particles, into pulses of electronic current. The time-integral of current, or charge (in coulombs), contained in each electronic pulse is a measure of the amount of energy (in joules, or equivalently, in electron-volts) deposited by the corresponding photon or charged particle.
The small-signal, linear-equivalent circuit model of the semi-conductor diode transducers used in these applications is shown conceptually as an idealized, pulsed current source connected in parallel with a capacitor which represents the diode junction capacitance. Depending on specific diode type and its operating (reverse-bias) potential, the junction capacitance can range from a few picofarads to several hundreds of picofarads.
When an X-ray photon, gamma-ray photon, or nuclear charged-particle is absorbed in the transducer, the energy from each impact is converted into a pulse of electronic current. Each pulse of electronic current, in turn, charges the transducer diode-junction capacitance producing a corresponding voltage signal whose amplitude equals the charge in the pulse divided by the diode junction capacitance.
The amount of charge in each transducer current pulse is quite small--on the order of a few femtocoulombs (1 femtocoulomb=10.sup.-15 coulomb) for low-energy X-ray photons--so that the corresponding voltage pulses are very weak and must be amplified by a factor of 10,000 or more, via several stages of amplification, to enable accurate signal processing, measurement, and analysis.
Applications such as radiation monitoring, dosimetry, and spectroscopy require detection, amplification, counting, measurement, and analysis of the distribution of amplitudes (referred to as "analysis of the pulse-height spectrum") of the electronic pulses produced by the transducer. Detection, counting, measurement, and analysis invariably occur in the presence of perturbing effects, such as thermal noise and electronic "shot" noise in the amplifying circuitry and in the transducer itself. These perturbing effects, in turn, set a lower limit to the magnitude of impulses which can be detected over the noise, and limit, as well, the overall precision of the amplitude-measurement and analysis process.
The lower limit of signal detectability and, correspondingly, the highest achievable measurement precision, is a function of various system components and operating parameters such as time constants, bias conditions, ambient temperature, etc. In order to preserve the highest possible signal-to-noise ratio, the first amplifying stage, or "preamplifier", used with a high-impedance, capacitive transducer, is generally configured as a "charge-integrating preamplifier" wherein the transducer, modelled by the parallel connection of pulsed current-source 11 and capacitor 12 shown in FIG. 1, is connected between the inverting input 16 of a high-gain inverting amplifying device 2, and circuit common 8.
A small feedback integrating capacitor 4, whose capacitance is typically of the order of five to ten percent of the transducer capacitance, is connected from the output of the amplifying circuit or device, back to the inverting input.
This configuration exploits the well-known "Miller-Effect" in which the additional parallel capacitance, as "seen" by the transducer, is effectively the value of the feedback capacitor multiplied by the voltage gain of the amplifying device. In the conceptual ideal, the amplifying device has "infinite" voltage gain, so that the additional parallel capacitance seen by the transducer is also "infinite". Thus, essentially all of the charge contained in each impulse of current generated by the transducer is deposited in the feedback capacitor rather than in the transducer capacitor. The voltage-signal amplitude at the output terminal of the preamplifier is given by the charge in each transducer pulse divided by the (actual) capacitance of the feedback capacitor.
As a practical matter, a discharge resistor 5 is connected in parallel with integrating capacitor 4 to prevent indefinite build-up of charge which would otherwise result in eventual saturation and circuit malfunction. A voltage signal V 17 at terminal 17 (shown as waveform 10 in FIG. 1) is approximated by a mathematical expression (referenced to time t=0): EQU V17=[Q/C4][1-e.sup.-t/tr ][e.sup.-t/tf ]
where: Q/C4 is the pulse amplitude, PA1 t.sub.r is the pulse "rise-time", which is governed by the charge-collection time-constant of the transducer combined with the bandwidth of amplifier 2, PA1 t.sub.f is the pulse "fall-time" defined by the time-constant t.sub.f =R5.times.C4
The foregoing is a description of the classic charge-integrating pre-amplifier of the prior art, the essential function of which is to cause most of the charge generated by a high-impedance, capacitive transducer to be deposited in an integrating capacitor which is substantially smaller than the transducer capacitance, thereby producing an amplified signal at the output. The preamplifier gain, expressed in units of "output volts per unit transducer charge" is nominally 1/C4.
The amplifying circuit or device in a classic charge-integrating pre-amplifier must be a high-gain, signal-inverting device, having low-noise, and high (essentially infinite) input-impedance. Above all, the amplifying circuit or device must be stable--free from any tendency toward self-oscillation. In order to achieve stable performance, a charge-integrating pre-amplifier of the prior art must be carefully crafted, utilizing premium, often costly components as justified in applications such as high-resolution x-ray and gamma-ray spectroscopy, working in conjunction with high-performance transducers to high standards of precision.
However, for many applications, particularly those applications requiring miniaturization, remote sensing over long distances, low operating voltage and/or very low power consumption, the classic charge-integrating preamplifier configuration of the prior art is too expensive, too bulky, and consumes too much power.